Recovery of alumina trihydrate during the Bayer process using scleroglucan

ABSTRACT

The invention provides methods and compositions for improving the production of alumina. The invention involves adding a product containing one or more polysaccharides to liquor within the fluid circuit of the production process, where one of the polysaccharides is scleroglucan. The use of scleroglucan can impart a number of advantages including at least some of: greater flocculation effectiveness, increasing the maximum effective dosage, faster settling rate. The production process can be a Bayer process.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent Ser. No.12/852,917, filed Aug. 9, 2010, issued as U.S. Pat. No. 8,252,266, thedisclosure of which is herein incorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

BACKGROUND OF THE INVENTION

The invention relates to a method for improving the Bayer process forthe production of alumina from bauxite ore. The invention concerns theuse of scleroglucan to improve the performance of unit operations withinthe Bayer process, specifically to enhance the settling of fine aluminatrihydrate crystals.

In the typical Bayer process for the production of alumina trihydrate,bauxite ore is pulverized, slurried with caustic solution, and thendigested at elevated temperatures and pressures. The caustic solutiondissolves oxides of aluminum, forming an aqueous sodium aluminatesolution. The caustic-insoluble constituents of bauxite ore are thenseparated from the aqueous phase containing the dissolved sodiumaluminate. Solid alumina trihydrate product is precipitated out of thesolution and collected as product.

As described at least in part, among other places, in U.S. Pat. No.6,814,873, the Bayer process is constantly evolving and the specifictechniques employed in industry for the various steps of the process notonly vary from plant to plant, but also are often held as trade secrets.As a more detailed, but not comprehensive, example of a Bayer process,the pulverized bauxite ore may be fed to a slurry mixer where aqueousslurry is prepared. The slurry makeup solution is typically spent liquor(described below) and added caustic solution. This bauxite ore slurry isthen passed through a digester or a series of digesters where theavailable alumina is released from the ore as caustic-soluble sodiumaluminate. The digested slurry is then cooled, for instance to about220° F., employing a series of flash tanks wherein heat and condensateare recovered. The aluminate liquor leaving the flashing operationcontains insoluble solids, which solids consist of the insoluble residuethat remains after, or are precipitated during, digestion. The coarsersolid particles may be removed from the aluminate liquor with a “sandtrap”, cyclone or other means. The finer solid particles may beseparated from the liquor first by settling and then by filtration, ifnecessary.

The clarified sodium aluminate liquor is then further cooled and seededwith alumina trihydrate crystals to induce precipitation of alumina inthe form of alumina trihydrate, Al(OH)₃. The alumina trihydrateparticles or crystals are then classified into various size fractionsand separated from the caustic liquor. The remaining liquid phase, thespent liquor, is returned to the initial digestion step and employed asa digestant after reconstitution with caustic.

Within the overall process one of the key steps is that of precipitationof the alumina trihydrate from the clarified sodium aluminate liquor.After the insoluble solids are removed to give the clarified sodiumaluminate liquor, also referred to as “green liquor”, it is generallycharged to a suitable precipitation tank, or series of precipitationtanks, and seeded with recirculated fine alumina trihydrate crystals. Inthe precipitation tank(s) it is cooled under agitation to induce theprecipitation of alumina from solution as alumina trihydrate. The fineparticle alumina trihydrate acts as seed crystals which providenucleation sites and agglomerate together and grow as part of thisprecipitation process.

Alumina trihydrate crystal formation (the nucleation, agglomeration andgrowth of alumina trihydrate crystals), and the precipitation andcollection thereof, are critical steps in the economic recovery ofaluminum values by the Bayer process. Bayer process operators strive tooptimize their crystal formation and precipitation methods so as toproduce the greatest possible product yield from the Bayer process whileproducing crystals of a given particle size distribution. A relativelylarge particle size is beneficial to subsequent processing stepsrequired to recover aluminum metal. Undersized alumina trihydratecrystals, or fines, generally are not used in the production of aluminummetal, but instead are recycled for use as fine particle aluminatrihydrate crystal seed. As a consequence, the particle size of theprecipitated trihydrate crystals determines whether the material is tobe ultimately utilized as product (larger crystals) of as seed (smallercrystals). The classification and capture of the different sizedtrihydrate particles is therefore an important step in the Bayerprocess.

This separation or recovery of alumina trihydrate crystals as product inthe Bayer process, or for use as precipitation seed, is generallyachieved by settling, cyclones, filtration and/or a combination of thesetechniques. Coarse particles settle easily, but fine particles settleslowly. Typically, plants will use two or three steps of settling inorder to classify the trihydrate particles into different sizedistributions corresponding to product and seed. In particular, in thefinal step of classification a settling vessel is often used to captureand settle the fine seed particles. Within the settling steps of theclassification system, flocculants can be used to enhance particlecapture and settling rate.

The overflow of the last classification stage is returned to the processas spent liquor. This spent liquor will go through heat exchangers andevaporation and eventually be used back in digestion. As a result, anytrihydrate particles reporting to the overflow in this final settlingstage will not be utilized within the process for either seed orproduct. Effectively such material is recirculated within the process,creating inefficiencies. Therefore, it is important to achieve thelowest possible concentration of solids in the overflow of the laststage of classification to maximize the efficiency of the process.

As described for example in U.S. Pat. No. 5,041,269, conventionaltechnology employs the addition of synthetic water soluble polyacrylateflocculants and/or dextran flocculants to improve the settlingcharacteristics of the alumina trihydrate particles in theclassification process and reduce the amount of solids in the spentliquor. While various flocculants are often used in the trihydrateclassification systems of Bayer plants, it is highly desirable to reduceas far as possible, the loss of solids with the spent liquor.

Thus there is clear need and utility for a method of improving theclassification and flocculation of precipitated alumina trihydrate inthe Bayer process. Such improvements would enhance the efficiency of theproduction of alumina from bauxite ore.

The art described in this section is not intended to constitute anadmission that any patent, publication or other information referred toherein is “prior art” with respect to this invention, unlessspecifically designated as such. In addition, this section should not beconstrued to mean that a search has been made or that no other pertinentinformation as defined in 37 CFR §1.56(a) exists.

BRIEF SUMMARY OF THE INVENTION

At least one embodiment of the invention is directed towards a methodfor settling alumina trihydrate in the Bayer process. The processcomprises adding to the system an effective amount of scleroglucan. Theuse of such a scleroglucan results in improved settling of aluminatrihydrate when compared to the use of conventional flocculants employedin this process.

At least one embodiment of the invention is directed towards a methodfor producing alumina comprising the addition of a compositioncontaining one or more polysaccharides, one of which is scleroglucan toliquor of a Bayer process fluid stream. The composition may be added tosaid liquor in a trihydrate classification circuit of said aluminaproduction process. The composition may be added to said liquor at oneor more locations in said process where solid-liquid separation occurs.The addition locations may facilitate inhibiting the rate of nucleationof one or more alumina trihydrate crystals in said process. The additionlocation may facilitate reducing the rate of scale formation in saidprocess. The composition may improve the yield of alumina trihydratesequestration.

At least one embodiment of the invention is directed towards acomposition comprising scleroglucan and Bayer liquor.

DETAILED DESCRIPTION OF THE INVENTION

For purposes of this application the definition of these terms is asfollows:

“Scleroglucan” is a polysaccharide consisting of beta-1,3-D-glucoseresidues with one beta-1,6-D-glucose side chain every three mainresidues

“Liquor” or “Bayer liquor” is liquid medium that has run through a Bayerprocess in an industrial facility.

In the event that the above definitions or a description statedelsewhere in this application is inconsistent with a meaning (explicitor implicit) which is commonly used, in a dictionary, or stated in asource incorporated by reference into this application, the applicationand the claim terms in particular are understood to be construedaccording to the definition or description in this application, and notaccording to the common definition, dictionary definition, or thedefinition that was incorporated by reference. In light of the above, inthe event that a term can only be understood if it is construed by adictionary, if the term is defined by the Kirk-Othmer Encyclopedia ofChemical Technology, 5th Edition, (2005), (Published by Wiley, John &Sons, Inc.) this definition shall control how the term is to be definedin the claims.

In at least one embodiment, a process for extracting alumina trihydratecomprises the digestion of pretreated bauxite ore in an alkaline liquorto produce a slurry of red mud solids and aluminate in suspension in thealkaline liquor then decanting the red mud solids from the alkalineliquor suspension to produce the decanting liquor; the passing of saiddecanting liquor through security filtration to remove all solids,precipitation and production of a slurry containing alumina trihydratesolids which then are flocculated and settled with the addition of apolysaccharide. Larger trihydrate particles are put through thecalcination process to produce purified alumina while finer particlesare re-used as seed for the precipitation process.

In at least one embodiment the preferred flocculant of the trihydratesolids in the process is scleroglucan or a blend of scleroglucan withone or more other polysaccharides such as dextran. The flocculant isadded in the range of 0.1 to 100 ppm. The most preferred dose range forthe flocculant is 0.1 to 10 ppm.

As described at least in U.S. Pat. Nos. 6,726,845, 3,085,853, 5,008,089,5,041,269, 5,091,159, 5,106,599, 5,346,628 and 5,716,530 and AustralianPatents 5310690 and 737191, polysaccharides such as dextran havepreviously been used in the Bayer Process. However, use of scleroglucanresults in superior and unexpected improvements in the activity whencompared to conventional polysaccharides or other reagents.

In at least one embodiment the composition is added to liquor in atrihydrate classification circuit of said alumina trihydrate productionprocess. The composition can be added to said liquor at one or morelocations in a Bayer process where solid-liquid separation occurs.

In at least one embodiment the composition can be added to said liquorat one or more locations in a Bayer process where it inhibits the rateof nucleation of one or more alumina hydrate crystals in said process.

In at least one embodiment the composition can be added to said liquorat one or more locations in a Bayer process where it reduces the rate ofscale formation in said process.

In at least one embodiment the composition can be added to said liquorat one or more locations in a Bayer process where it facilitates red mudclarification in the process.

In at least one embodiment the composition can be added in combinationwith or according to any of the compositions and methods disclosed incommonly owned and at least partially co-invented co-pending patentapplication having a title of “THE RECOVERY OF ALUMINA TRIHYDRATE DURINGTHE BAYER PROCESS USING CROSS-LINKED POLYSACCHARIDES.”

EXAMPLES

The foregoing may be better understood by reference to the followingexamples, which are presented for purposes of illustration and are notintended to limit the scope of the invention.

Within the examples given below a range of solutions containingscleroglucan and dextran in various ratios were used. The performance ofthese blends was compared to the performance of scleroglucan and/ordextran when used alone. The compositions of the combined formulationstested are given in table 1. When doses of such blends are quoted, thisrefers to the combined amount of scleroglucan plus dextran added to theprocess.

TABLE 1 Ratio of components in blended formulations used within theexamples Ratio of Formulation Scleroglucan:Dextran (I) 1:7 (II) 8:7(III) 1:6 (IV) 2:5

Example 1

Secondary thickener (ST) overflow from an operating Bayer plant wascollected just prior to the test, divided into 1 L aliquots in clear 1 Lmeasuring cylinders and placed in a waterbath at 75° C. Each cylindercontained approximately 83 g/L of alumina trihydrate. The productstested were added as dilute solutions one after another to the surfaceof the slurry and mixed well using a gang plunger. The settling rate wasmeasured by recording the time taken for the solids interface to reachthe 600 mL mark of the cylinder from when the mixing ceased. The resultof the settling rate is converted to meters per hour (m/hr) in Table 2.

TABLE 2 Alumina trihydrate settling results for Example 1. Settling RateFormulation Dose (ppm) (m/hr) Dextran 0.7 2.46 Scleroglucan 0.7 3.94

The data in table 2 indicate that a significantly faster settling ratecan be achieved with scleroglucan as the flocculant compared to anequivalent dose of dextran.

Example 2

The same method as that used in example 1 was employed. The onlydifference was the solids content of this slurry collected was 45 g/L.After settling the samples were left to settle for 15 minutes followedby removal of 50 mL of slurry from the surface of the slurry using asyringe. This aliquot was filtered through a pre-weighed Supor®-450membrane filter paper. Solids were then washed with hot deionized waterand dried at 100° C. The filter paper and solids were then reweighed andthe mass of solids calculated. This mass is listed as “overflow solids(g/L)” in Table 3. The results are displayed in Table 3 and again showthe increase in settling rate when scleroglucan is used or included incombination with dextran in a formulation. Additionally, superior(lower) overflow solids are observed when scleroglucan or formulationscontaining scleroglucan are used.

TABLE 3 Alumina trihydrate settling results for Example 2. Settling RateOverflow Formulation Dose (ppm) (m/hr) Solids (g/L) Dextran 0.7 4.860.80 Scleroglucan 0.7 5.40 0.78 (I) 0.8 6.57 0.71

Example 3

The same method as in example 2 was used. The solids content of theslurry collected for this test was 67 g/L.

TABLE 4 Alumina trihydrate settling results for Example 3. Settling RateOverflow Formulation Dose (ppm) (m/hr) Solids (g/L) Dextran 0.7 4.260.76 (I) 0.8 5.28 0.70

Example 4

The same method as in example 2 was used. Two separate sets of data werecollected in two experimental runs. The solids content of the slurry forthe individual runs in this example was 79 g/L in both cases. The top 50mL of the slurry was sampled after 10 minutes of settling instead of 15minutes as in example 2.

TABLE 5 Alumina trihydrate settling results for Example 4 run 1 SettlingRate Overflow Formulation Dose (ppm) (m/hr) Solids (g/L) Dextran 0.73.47 0.78 Dextran 0.7 3.63 0.87 (I) 0.64 4.42 0.87 (I) 0.8 4.58 0.89

TABLE 6 Alumina trihydrate settling results for Example 4 run 2 SettlingRate Overflow Formulation Dose (ppm) (m/hr) Solids (g/L) Dextran 0.353.33 1.17 Dextran 0.7 4.54 1.01 (II) 0.38 5.65 1.03 (II) 0.75 7.97 0.67

Example 5

Bayer plant spent liquor (200 mL) and air dried plant seed (16 g) wascombined in a bottle and heated to 65° C. in a rotating water bath. Oncethe slurry had reached equilibrium it was transferred to a 250 mLmeasuring cylinder that was suspended in a water bath at 65° C. Theslurry was then dosed with product, mixed thoroughly and allowed tosettle for three minutes followed by removal of 50 mL of slurry from thesurface of the slurry using a syringe. This aliquot was filtered througha pre-weighed Supor®-450 membrane filter paper. Solids were then washedwith hot deionized water and dried at 100° C. The filter paper andsolids were then reweighed and the mass of solids calculated. This massis listed as “overflow solids (g/L)” in Table 7.

TABLE 7 Alumina trihydrate settling results for Example 5. OverflowTreatment Dose (ppm) Solids (g/L) Undosed 0 1.98 Dextran 0.35 1.17Dextran 0.70 1.04 (I) 0.40 1.10 (I) 0.80 0.86

Example 6

The same method as in example 5 was used in this example except that 500ml of liquor and 40 g of seed was used for each treatment. The samplingof the slurry was conducted after 5 minutes of settling time. Thesettling rate was measured by the time taken for the solid interface toreach the 350 mL graduation on the cylinder once mixing had ceased.

TABLE 8 Alumina trihydrate settling results of Example 6. Settling RateOverflow Treatment Dose (ppm) (m/hr) Solids (g/L) Untreated 0 1.04 4.41Dextran 0.7 1.29 2.71 (I) 0.7 1.57 2.71 (II) 0.7 2.05 2.51

Example 7

The same method as in example 6 was used except the solids content ofthe slurry in this example was increased to 120 g/L.

TABLE 9 Alumina trihydrate settling results for Example 7. Settling RateOverflow Treatment Dose (ppm) (m/hr) Solids (g/L) Untreated 0 0.84 4.97Dextran 0.7 1.45 2.79 Dextran 1.4 1.59 2.50 (I) 0.7 1.58 2.78 (I) 1.41.94 1.86 (III) 0.7 1.56 2.59 (III) 1.4 1.80 1.89 (IV) 0.7 1.73 2.57(IV) 1.4 2.14 1.91 Scleroglucan 0.7 1.99 2.01 Scleroglucan 1.4 2.48 1.39

Example 8

Plant spent liquor (1 L) and air dried plant seed (80 g) was combined ina bottle and heated to 65° C. in a rotating water bath. Once equilibriumwas established the slurry was dosed with flocculant (as appropriate)mixed well and poured into a 1 L Imhoff cone. The slurry was allowed tosettle in the cone for twenty minutes before allowing the slurry todischarge through the bottom hole. The discharge time was measured fromwhen the plug was removed after the twenty minutes of settling to whenall the contents of the cone had been discharged.

TABLE 10 Example 8 discharge times for settled alumina trihydrateslurries using Imhoff cones. Imhoff Cone Treatment Dose (ppm) dischargetime (seconds) Untreated 0 66 Dextran 0.7 31 Scleroglucan 0.7 30 (I) 0.433

Example 9

The same method as in example 8 was used but the slurry in this examplewas plant secondary classification overflow slurry collected from theplant just prior to the test. The solids content of this slurry was 62g/L.

TABLE 11 Example 9 discharge times for settled alumina trihydrateslurries using Imhoff cones. Imhoff Cone Treatment Dose (ppm) dischargetime (seconds) Untreated 0 136 Dextran 0.7 40 Scleroglucan 0.7 11 (I)0.8 23

Example 10

The same method as in example 9 was used in this example. The solidscontent of this slurry was 100 g/L.

TABLE 12 Example 10 discharge times for settled alumina trihydrateslurries using Imhoff cones. Imhoff Cone Treatment Dose (ppm) dischargetime (seconds) Untreated 0 127 Dextran 0.7 114 (I) 0.8 112

The results from examples 8, 9 and 10 indicate that scleroglucan has anunexpected impact on the flow of settled alumina trihydrate solids.

When applied at the same dose rates as dextran, scleroglucan provides afaster settling rate, more desirable rheological properties in thesettled bed and maintains similar or better performance in overflowclarity. The use of scleroglucan is effective when applied either alone,or as a blend with other polysaccharides such as dextran.

While this invention may be embodied in many different forms, there areshown in the drawings and described in detail herein specific preferredembodiments of the invention. The present disclosure is anexemplification of the background and principles of the invention and isnot intended to limit the invention to the particular embodimentsillustrated. All patents, patent applications, scientific papers, andany other referenced materials mentioned anywhere herein, areincorporated by reference in their entirety. Furthermore, the inventionencompasses any possible combination of some or all of the variousembodiments described herein and incorporated herein.

The above disclosure is intended to be illustrative and not exhaustive.This description will suggest many variations and alternatives to one ofordinary skill in this art. All these alternatives and variations areintended to be included within the scope of the claims where the term“comprising” means “including, but not limited to”. Those familiar withthe art may recognize other equivalents to the specific embodimentsdescribed herein which equivalents are also intended to be encompassedby the claims.

All ranges and parameters disclosed herein are understood to encompassany and all subranges subsumed therein, and every number between theendpoints. For example, a stated range of “1 to 10” should be consideredto include any and all subranges between (and inclusive of) the minimumvalue of 1 and the maximum value of 10; that is, all subranges beginningwith a minimum value of 1 or more, (e.g. 1 to 6.1), and ending with amaximum value of 10 or less, (e.g. 2.3 to 9.4, 3 to 8, 4 to 7), andfinally to each number 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 containedwithin the range.

This completes the description of the preferred and alternateembodiments of the invention. Those skilled in the art may recognizeother equivalents to the specific embodiment described herein whichequivalents are intended to be encompassed by the claims attachedhereto.

1. A composition comprising: scleroglucan and Bayer liquor.
 2. The composition of claim 1, wherein the scleroglucan is present at a concentration of 0.1 to 100 ppm.
 3. The composition of claim 1, wherein the scleroglucan is present at a concentration of 0.1 to 10 ppm.
 4. The composition of claim 1, wherein the composition further comprises a polysaccharide.
 5. The composition of claim 4, wherein the polysaccharide is dextran.
 6. The composition of claim 5, wherein the scleroglucan and the dextran are present at a scleroglucan:dextran weight ratio range of 1:7 to 8:7.
 7. The composition of claim 1, wherein the composition further comprises at least one solid.
 8. The composition of claim 1, wherein the composition further comprises alumina trihydrate crystals.
 9. The composition of claim 1, wherein the composition further comprises red mud.
 10. A composition consisting essentially of: scleroglucan and Bayer liquor. 